1. Field of the Invention
The present invention relates to a blown film process and apparatus for making plastic film bags. The process comprises producing a film tubing by extrusion, and expanding the tubing by controlled internal and external excess pressure to obtain a film bag which expands increasingly from a smallest diameter at the extrusion die via a transition point to a maximum diameter established by a guide ring spaced from the die. Subsequently, the bags are flattened and rolled onto a drum.
2. Description of the Prior Art
A decisive problem in connection with blown film processes, in particular with the preparation of high molecular weight, low pressure polyethylene sheets, is the adjustment and maintenance of stable process conditions. The quality and uniformity of the produced film is dictated, for instance, by the selection and composition of the catalyst components, the polymerization medium, the pressure, temperature and extrusion speed parameters, and by the constructional design of the extruder and its die. Only a limited range of suitable values for these parameters will result in stable operating conditions. This acceptable tolerance range often is exceeded in practice, particularly in the preparation of paper-like sheets of high molecular weight, low pressure polyethylene. Thus during the manufacture of such sheets, frequent machine down time is experienced, or a low quality product is produced.
These difficulties are due, inter alia, to the following reasons:
1. A knowledge of the temperature and/or pressure of the extruder die itself is not sufficient to characterize the behavior of the melt during inflation after leaving the extruder die. Thus control of these extruder parameters does not guarantee that the extruded plastic tubing will undergo uniform inflation with time, even for a specific material quality.
2. It is not possible conclusively to determine the viscosity of the melt from its temperature.
3. In a production process, the amount and temperature of cooling air not only establishes the rate at which heat is removed from the surface of the sheet bubble, but also, in the preparation of paper-like sheets, greatly influences the similarity to paper, the variation of the wall thickness, and the resistance to expansion of the sheet in both longitudinal and transverse directions.
4. Even with a constant extruder output and a constant sheet take-off rate, the weight per square meter of the produced sheets, and accordingly, the wall thickness, are not necessarily constant. This is due to the fact that even small changes in temperature at the intake region of the extruder may result in substantial variation in the amount of extruder output.
5. In typical prior art systems the width of the flattened sheet or bag is measured just before winding onto the drum. This width measurement, however, does not lend itself to use as a control input for adjusting the process parameters, because the transit time from the extruder to the winding device is too long. The process parameters which caused the measured width variation may already have so exceeded the acceptable tolerance that correction is difficult or impossible.
6. Even to this date, there is insufficient knowledge about the relationship between the properties of the raw material and the resultant conditions prevailing during manufacture so as to be able to stabilize or control the production process and the quality of the produced sheet as a function of the raw material characteristics. The result found for a specific raw material cannot be transferred to form the basis for calculations with respect to other raw materials.
It is an object of the invention, in connection with a known blown film process, to control the conditions resulting in stability of the produced film bag so as to timely counteract an imminent impairment of the film quality by appropriate measures. It is also an object of the invention to establish the most favorable process parameters for different machines and/or melt materials by observing and controlling the shape of the film bag being formed.